1. Field of the Invention
The present invention relates to a device for the removal and deposition of loads between two supports in repeated relative vertical motion.
The invention applies more particularly to the removal of loads resting on the deck of a ship in the course of trans-shipment operations at sea, and notably in the provisioning of marine platforms situated off-shore.
2. Description of the Prior Art
The embarkment of packages and various loads for drilling and constructional operations at sea is always a difficult operation, especially in bad weather, when the relative movements of the ships generated by the swell are considerable, the relative vertical movements being called "heaving" in the following description.
At present, in heavy weather, in the handling of the removal of a load from a supply ship, it frequently happens that in hoisting under the effect of the swell, the deck of the ship strikes the load from below after it has already been raised from the deck and this in spite of high lifting speeds obtainable today with modern cranes and the high skill of crane operators who cannot always unfortunately avoid accidents sometimes very serious (loss of the packages or breaking of the crane jib), or even fatal accidents.
For the operation to be carried out without mishap, the load should be lifted from the deck of the ship when the latter reaches the vicinity of the crest of the heaving movement, which necessitates starting the lifting a certain time beforehand, on the one hand to take into account the inertia of the crane to be put into motion and, on the other hand, to take up the slack of the slings attached to the packages and tighten them. Now the crane operator generally experiences great difficulty in estimating and sensing the limited period of time during which he can start-up the lifting of the load, since:
the crane cannot, on account of its limited speed and its own inertia, constantly follow the movement due to the swell by keeping the slings taut,
the crane operator cannot correctly evaluate, taking into account its position, the heaving of the ship which occurs vertically with respect to him,
the sling of the package being slack at the beginning of the operation to permit attachment, it must be tightened before being able to lift the package, this tightening time being of the same order of magnitude as the period of the swell, and the crane operator cannot very well evaluate in advance the length of cable that he will have to wind in, so that he runs the risk of not terminating his sling tautening operation at the moment of the crest of the swell.
Progress has been made with the pulley-block described in French Pat. No. 1,509,895 and which includes a load cable tensioning device; the latter ensures compensation for the heaving and permanent tension in the slings before the raising of the load which is obtained by braking the unwinding of the cable. However, since the blocking of the brake is carried out manually at a moment judged favorable by the crane operator, the success of the operation depends entirely on the skill of the latter and if the braking order is given when the support for the load is lifted, the slings and the cable take up the slack and are suddenly tightened when the support drops, which is very dangerous and can result in accidents.
In addition, in the course of lifting, taking into account the irregular movements in the plane of the supply ship, and due to the fact that the crane operator cannot simultaneously take care of the tightening of the sling and the lifting of the package and in addition ensure that the crane jib head remains well vertically aligned over the load, it happens fairly frequently that the load is raised at a slant with a cable pulling at a considerable angle with respect to the vertical.
There then occurs ripping of the loads, collisions, danger of the packages catching one another or in the bulwarks of the supply ship and in the limit, there is a risk of twisting the crane jib which is not constructed to withstand considerable lateral traction forces.
To overcome these aforementioned drawbacks, other flexible devices of the damper electrical pulley-block jack or electrical cable type have already been tried but, hooked to the crane hook or installed on the crane itself, they have the major drawback of having much too limited a travel which does not permit the manipulations to be followed as soon as the amplitude of the heaving increases even rather little, although the general environmental conditions may still permit operating with full safety; in addition, this equipment is rather fragile, which is incompatible with the severe conditions under which they are often obliged to operate.
It is therefore an object of the present invention to provide a device for the removal and deposition of loads between two supports in relative heaving movement without a risk of impact against the packages or of catching the latter on one another or on the bulwarks of the ship, and this up to the limit of operational possibilities in a heavy swell.